It was a sharp reversal from the first quarter of 2015, when investors piled into oil ETPs trying to position for a rebound when production numbers start to dip.

But with Saudi Arabia pumping at near-record highs in an attempt to win a battle for market share against U.S. shale production, crude supply continues to outweigh demand.

Towards the end of March there were signs that investor fatigue had set in, as negative roll yields eroded profits. The April rally then gave some investors an opportunity to cash in.

“With the fundamental outlook for oil not improving significantly, outflows are likely short-term investors taking money off the table after a nice recovery in oil prices since late March,” Ursula Marchioni, head of ETP research at BlackRock’s iShares EMEA, said.

“Oil ETPs are not ideal for longer-term bets due to roll costs eroding returns and the market has been in contango ever since the ETP flows began to pick up late last year,” she added.

Energy was the best-performing sector in the S&P GSCI in March, up 17.4 percent, with all petroleum commodities reporting double-digit gains. But the persistent weakness in physical crude markets suggests oil futures may sell off again.

Nitesh Shah, commodities research associate at ETF Securities, an issuer of ETPs, said outflows from oil ETPs had continued into May. “The rally was a bit premature because global oil supply hasn’t really tightened yet,” he said.

He suggested that some investors had taken profits in April in case OPEC chooses not to cut production at its June meeting, and prices fall again.

Gold ETPs had modest inflows in April after outflows of over $2 billion in March. Shah partly attributed this reversal to nervousness around the outcome of Greece’s debt negotiations.